enter in something
to enroll as a participant in something, such as a contest, competition, etc. She was not ready to enter in the competition. I can't enter in that contest. I'm not prepared.
enter someone or something in(to) something
to enroll someone or something in something; to make someone or something a competitor in something. I will enter you into the contest whether you like it or not. The trainer entered his fastest horse in the race.